Recently, waveguides are vigorously developed which convert propagating light into near-field light. Application of the waveguides is actively proposed not only for an optical interconnection of an optical circuit but also for a recording head and a recording device. In the field of optical recording, miniaturization of an optical spot is advanced for a higher recording density. In view of this, use of near-field light is proposed. In the use of near-field light, a high intensity of near-field light is required for a high S/N ratio. For this reason, a waveguide is used which converts propagating light into near-field light by, particularly, a surface plasmon polariton technology.
For optically-assisted magnetic recording, it is necessary to take into consideration relative positions of a waveguide, a magnetic pole, and a reproducing element.
For example, in an optically-assisted magnetic recording head disclosed in Patent Literature 1, a constriction having a rectangular shape or a V-shape is formed in a metal film so that a position of a magnetic field generated due to an electric current passed through the metal film may be matched with a position of near-field light generated due to light which enters the constriction. In this case, a polarization direction of the incident light is a direction parallel with a longitudinal direction of the constriction.
In each of optically-assisted magnetic recording heads disclosed respectively in Patent Literatures 2 and 3, a waveguide whose cross-section is a triangle is formed, and a polarization direction is adopted which is perpendicular to that one side of the triangle which is oriented toward a main magnetic pole. This causes near-field light to localize at a position on the one side of the triangle, and generates near-field light in the vicinity of a main magnetic pole.
In an optically-assisted magnetic recording head disclosed in Patent Literature 4, a metal film having an aperture at an output terminal of a semiconductor laser so that near-field light may be generated, by the metal film, through the use of surface plasmon polariton enhancement. Patent Literature 4 exemplifies a triangle as one example of a shape of the aperture. In this case, a polarization direction of incident light is a direction perpendicular to one side of the triangle.
In each case, light incident on the metal film is converted into surface plasmon polaritons which are a kind of near-field light. The surface plasmon polaritons propagate along a tip of the constriction and one side of the triangle, thereby reaching the exit surface.
Non-patent Literature 1 teaches that in a case where light polarized in a width direction of a V-shape of a V-shaped waveguide enters the V-shaped waveguide, generated surface plasmon polaritons converge at a tip of the V-shape.
The following describes this, with reference to (a) through (d) of FIG. 15. (a) of FIG. 15 is a perspective view illustrating an arrangement of the waveguide disclosed in Non-patent Literature 1. (b) of FIG. 15 is a cross-sectional view illustrating the waveguide in (a) of FIG. 15 which cross-sectional view is parallel with an X-Y plane. (c) of FIG. 15 is a cross-sectional view illustrating the waveguide in (a) of FIG. 15 which cross-sectional view is parallel with a Y-Z plane and in which propagation of surface plasmon polaritons is illustrated. (d) of FIG. 15 is a diagram illustrating the propagation illustrated in (c) of FIG. 15.
As illustrated in (a) of FIG. 15, X, Y, and Z axes are assumed. A waveguide 100 consists of a metallic member 101 and a dielectric member 102. The metallic member 101 has a groove whose cross-section parallel with an X-Y plane is a V-shape. The dielectric member 102 is provided in the groove.
As illustrated in (b) of FIG. 15, a width, in a direction of an X-axis, of the groove formed in the metallic member 101 (i.e., a width of the dielectric member 102) becomes narrower from a plus direction of a Y-axis to a minus direction of the Y-axis. The narrower the width in the direction of the X-axis, the larger the effective refractive index for surface plasmon polaritons excited in a case where light polarized in the direction of the X-axis enters the waveguide 100. In this case, a track of the surface plasmon polaritons propagating through the groove in the metallic member 101 is indicated by an arrow A in (c) of FIG. 15. That is, the surface plasmon polaritons change their propagation direction toward a tip of the groove.
If incident light travels from a medium having a small refractive index to a medium having a large refractive index, θ4<θ3 is satisfied by Snell's law, as illustrated in (d) of FIG. 15. Since a groove of a V-shaped waveguide such as the waveguide 100 is considered to be a group of layers in which a refractive index gradually changes, the surface plasmon polaritons propagating through the groove in the metallic member 101 converge at the tip of the V-shape of the groove.
As indicated by a dashed line in (d) of FIG. 15, usually, light (surface plasmon polaritons) is reflected on an interface between two media which are different in refractive index from each other. However, if a difference between respective refractive indexes of the two media is very small, a reflectance is very small. That is, decreasing an angle of an opening of the V-shape of the groove makes it possible to decrease a change in effective refractive index. As a result, reflection of the light can be suppressed in the V-shaped waveguide 100 so that the surface plasmon polaritons may be converged at a Z-axis (i.e., at the edge of the groove).